In 1957, following a seminar Finkelstein gave in London, he met Roger Penrose, then a graduate student from Cambridge. The seminar, on extending Schwarzschild's metric, a basic ingredient of the current understanding of black holes, had been a revelation to Penrose. After the seminar Penrose explained to Finkelstein his spin-networks research, and the two men exchanged their research subjects for ever after.
In 1958, at age 29, Finkelstein described what is now known as a black hole, or “unidirectional membrane” as he described it. In essence, Finkelstein determined that whatever falls past the Schwarzschild radius into a black hole cannot escape it; the membrane is one-directional. This important work influenced the decisions of Lev Davidovich Landau, Roger Penrose and John Archibald Wheeler to accept the physical existence of event horizons and black holes and was instrumental in bringing general relativity into mainstream physics, encouraging today’s vibrant black hole research.
The 2015 observation of gravitational waves represents one of the more recent discoveries to benefit from the earlier work of Finkelstein and other physicists who sought to understand the nature of the universe.
David was one of my greatest friends and a genuine hero to me. In recent years he and I did not have much opportunity to see each other, but just the fact that he was there was an inspiration. Whenever I felt frustrated by my colleagues it was a great source of comfort to think of David's shining intellect, his curiosity, and his integrity. It is hard to accept that he is gone.
I learned many things from David that played major roles in my thinking… When I was a young physicist quantum field theory meant Feynman diagrams. It was the great work of Finkelstein and Rubinstein on topology in [quantum field theory] that first made me understand the richness of the subject. It took quite a few years for the rest of the world to catch up.
The great thing was that I got to hear about it straight from David himself. For a long time I have been focused (some might say obsessed) on black holes. Again, it was David who first understood the nature of the horizon. That was a good deal before I knew him, but when I first became interested in gravity (1967 or so) it was David who said "Forget perturbation theory. Black holes are the key."
In fact almost every time I start thinking about something new, I recall something seminal that David said.
Leonard Susskind, Stanford University